1. Field of the Invention
The present invention relates to a process for producing magnetic metal particles suitable for magnetic recording mediums.
2. Description of Related Art
In recent years, magnetic recording mediums have been more increasingly required to be improved so as to have higher recording density and higher performance. As associated with such a trend, an increasing interest has been directed to iron or iron-based magnetic metal particles (referred to as magnetic metal particles hereunder) having higher saturation magnetization and higher coercive force than those of acicular magnetite and maghemite, even compared with those of cobalt modified magnetic iron oxides of the magnetite and maghemite, as magnetic particles to be used in magnetic recording mediums. The magnetic metal particles have greater promise for use in higher performance magnetic recording mediums such as high image quality video tapes for HDTV in recent years and high recording density disks as well as for practical use in digital audio tapes and 8 mm video tapes.
There has been recently a need for fine magnetic metal particles useful for high performance magnetic recording mediums in the acicular form of uniform size normally having a largest dimension of about 0.3 .mu.m, or even no larger than 0.2 .mu.m which do not contain any sintered agglomerates and which are more excellent in dispersibility in magnetic paint systems, orientability and filling property in magnetic coating films with the paint systems, and flatness of the coatings. Above all, for the purpose of providing a recording medium having a higher output level with a reduction in the noise level attributable to particle size, further ultra-fine particles have been required so that an increasing attention has been directed to acicular magnetic metal particles of smaller size having excellent magnetic properties and aging stability.
Magnetic metal particles have been produced generally by a method where hydrated iron oxides of the acicular form such as goethite are dehydrated under heat to produce iron oxide particles such as hematite which are reduced under heat in a reducing gaseous atmosphere such as hydrogen to produce magnetic metal particles. However, the thermal process, especially the reduction step, tends to cause sintering of particles and deformation of particle shape. Such deformation of particle shape and the coarsening of .alpha.-Fe crystalline particles impair significantly magnetic characteristics of the end magnetic metal particles. This has an influence on the production of fine magnetic metal particles in the manner that the finer the starting material, hydrated iron oxide particles, the greater the tendency of deformation of particle shape, making it difficult to achieve the desired levels of magnetic characteristics and aging stability.
Various techniques have been proposed heretofore to overcome the aforementioned problems. For example, one of well known techniques comprises treating the surfaces of the hydrated iron oxide particles and iron oxide particles with so-called shape-retaining agents such as silicon compounds, boron compounds, aluminum compounds and zirconium compounds, and thereafter dehydrating the treated particles under heat to reduce them.
Although the aforementioned treatment, for example, with silicon compounds, boron compounds and phosphorus compounds results in desired effects of preventing the sintering and the deformation of particle shape to facilitate the production of magnetic metal particles having good magnetic characteristics, it tends to inhibit proceeding of the reduction or impair the dispersibility in the course of the production of magnetic recording mediums so that adverse affections on magnetic characteristics, audio and video characteristics of the magnetic recording mediums may be observed.
With aluminum compounds, a good compatibility of the treated particles with paint resins and solvents to be used for the production of magnetic recording mediums results in desired effects in dispersibility. However, the aluminum compounds are likely to be taken into the particle matrix during the thermal dehydration treatment so that a reduction-inhibiting action may occur, or the effects of preventing sintering and retaining particle form may be diminished in the course of the thermal reduction.
On the other hand, with zirconium compounds, though the effects of preventing the sintering and deformation of shape as described above can be achieved to some degree, optimization of the effects requires increasing the amount of zirconium compounds to be deposited, which in turn tends to prevent the proceeding of the reaction in the reduction treatment process and diminish the magnetic characteristics such as saturation magnetization of the produced magnetic metal particles. At any rate, not a few problems remain to be solved.